1. Field of the Invention
This invention relates to processes for hydrocarbon processing. In another aspect, this invention relates to processes for the removal of contaminants from hydrocarbons. In still another aspect, this invention relates to processes for the removal of butadiene derivative contaminants from a cracked hydrocarbon stream.
2. Description of the Prior Art
Cracking is a well known process involving decomposition and molecular recombination of organic compounds, especially hydrocarbons obtained by means of heat to form molecules suitable for motor fuels, monomers, petrochemicals, etc. A series of condensation reactions takes place accompanied by transfer of hydrogen atoms between molecules which brings about fundamental changes in their structure. Methods of hydrocarbon cracking include thermal cracking, which utilizes heat to carry out the cracking, and catalytic cracking, which utilizes a catalyst generally either with the moving-bed or fluid-bed technique.
Steam cracking is widely used for the production of light olefins from saturated hydrocarbons. Reaction conditions for steam cracking of saturated hydrocarbons are selected to maximize the production of light olefins. Typically, cracking is practiced at a weight ratio of 0.3:1.0 of steam to hydrocarbon with the reactor coil outlet at 760.degree.-870.degree. C., and slightly above 100 kPa (atmospheric) pressure.
The type of feedstocks and the reaction conditions determine the mix of hydrocarbon products produced. Many steam crackers operate on light paraffin feeds consisting of ethane and propane and the like. However, a significant amount of steam cracking capacity operates on feedstocks which contain propane and heavier compounds. Steam cracking such feedstocks produces a hydrocarbon mixture composed of many marketable products, notably propylene, isobutylene, butadiene, amylene and pyrolytic gasoline.
Generally, in steam cracking, the cracked gases emerging from the reactors are rapidly quenched to arrest undesirable secondary reactions which destroy light olefins. Unfortunately, in addition to the foregoing desirable components, undesirable acetylenic compounds, are many times also produced. These acetylenic compounds generally are required to be removed at least to the level of a few parts per million in order for the stream to meet process requirements, for example, in polymerization processes or to avoid formation of explosive metal acetylides in equipment. Typically these acetylenic compounds are alpha-acetylenes corresponding to olefins and diolefins that were present in the steam cracking, the most common of which are vinyl acetylene, methyl acetylene and ethyl acetylene.
If allowed to remain in the hydrocarbon mixture, these acetylenic compounds may cause fouling of the equipment, interfere with polymerization reactions, and in some cases pose safety hazards. It is, therefore, highly desirable to remove these acetylenic compounds from the hydrocarbon mixture.
In hydrocarbon processing it is known that the acetylenic compounds may be removed by distillation. It is also known that the acetylenic compounds can be selectively hydrogenated and thereby removed from a hydrocarbon stream by passing a mixture of the hydrocarbon with hydrogen over a catalyst of moderate activity, for example, a copper catalyst. The location and complexity of a typical hydrogenation unit is set by the compatibility of process conditions with the catalyst system used for the selective hydrogenation of these contaminants. Typical hydrogenation units required for the production of marketable distillation products include, in addition to the acetylene converter which treats the C.sub.2 stream, a methylacetylene/propadiene converter ahead of the C.sub.3 splitter to remove contaminants from propylene and propane products and to avoid the risk of detonation in the C.sub.3 splitter caused by build-up of methylacetylene and propadiene; a hydrogenation unit ahead of the debutanizer to remove alpha acetylenes from C.sub.4 and C.sub.5 olefins; a hydrogenation unit on the debutanizer overhead to remove alpha acetylenes from C.sub.4 olefins; and either a heat soaker or a hydrogenation unit on the debutanizer bottoms to remove additional C.sub. 5 acetylenes from pyrolysis gasoline.
Generally, hydrocarbon feeds which contain the aforementioned acetylenic contaminants are introduced to a hydrogenation unit wherein they are reacted with hydrogen under conditions of temperature, pressure and over a catalyst selective for the hydrogenation of the contaminants contained therein. Catalysts suitable for use in hydrogenating acetylenic contaminants are described in U.S. Pat. Nos. 3,076,858, 3,327,013, and 4,101,451, all herein incorporated by reference.
While hydrogenation may help eliminate undesirable acetylenic compounds, other undesirable compounds are produced as by-products during hydrogenation.
Specifically, in a butadiene recovery unit, the first step is generally to hydrogenate the acetylenic contaminants in a hydrogenation unit. Side reactions in the hydrogenation unit produce other undesirable compounds that are known collectively as "green oil."
"Green oil" refers to a mixture of compounds produced in the hydrogenation of a butadiene containing hydrocarbon mixture and is known to contain oligomers of butadiene, sometimes referred to as dimers and trimers, and may contain material having up to 16 or more carbon atoms per molecule.
In conventional hydrocarbon processing methods for butadiene recovery, the butadiene and other C.sub.4 hydrocarbons are generally separated from the green oil by fractional distillation in a green oil debutanizer tower. This green oil or debutanizer tower represents a major investment item in a butadiene recovery unit in terms of its initial cost of construction, as well as its cost of maintenance and operation.
It would, therefore, be highly desirable to have a hydrocarbon processing method for butadiene recovery wherein the need for a green oil or debutanizer tower is eliminated.